are of no direct use to their possessors. Physical conditions probably
have had some little effect on structure, quite independently of any
good thus gained. Correlation of growth has no doubt played a most
important part, and a useful modification of one part will often have
entailed on other parts diversified changes of no direct use. So again
characters which formerly were useful, or which formerly had arisen
from correlation of growth, or from other unknown cause, may reappear
from the law of reversion, though now of no direct use. The effects of
sexual selection, when displayed in beauty to charm the females, can be
called useful only in rather a forced sense. But by far the most
important consideration is that the chief part of the organisation of
every being is simply due to inheritance; and consequently, though each
being assuredly is well fitted for its place in nature, many structures
now have no direct relation to the habits of life of each species.
Thus, we can hardly believe that the webbed feet of the upland
goose or of the frigate-bird are of special use to these birds; we
cannot believe that the same bones in the arm of the monkey, in the
fore leg of the horse, in the wing of the bat, and in the flipper of
the seal, are of special use to these animals. We may safely attribute
these structures to inheritance. But to the progenitor of the upland
goose and of the frigate-bird, webbed feet no doubt were as useful as
they now are to the most aquatic of existing birds. So we may believe
that the progenitor of the seal had not a flipper, but a foot with five
toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse, and
bat, which have been inherited from a common progenitor, were formerly
of more special use to that progenitor, or its progenitors, than they
now are to these animals having such widely diversified habits.
Therefore we may infer that these several bones might have been
acquired through natural selection, subjected formerly, as now, to the
several laws of inheritance, reversion, correlation of growth, etc.
Hence every detail of structure in every living creature (making some
little allowance for the direct action of physical conditions) may be
viewed, either as having been of special use to some ancestral form, or
as being now of special use to the descendants of this form either
directly, or indirectly through the complex laws of growth.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
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